Sequence stratigraphic evolution and cyclicity of an ancient coastal desert system: The pennsylvanian-permian lower cutler beds, paradox basin, Utah, U.S.A.

摘要

The lower Cutler beds comprise a 200-m-thick succession of at least 12 repeating cyclic packages of strata, each 10-18 m thick, that are of mixed eolian, fluvial, and shallow marine origin. These accumulated during the latest Pennsylvanian to early Permian in the Paradox foreland basin of southeast Utah. Each depositional cycle comprises a lower unit composed of a bioclastic wackestone or a bioclastic sandy calcarenite facies, both of shallow marine origin. The upper units in each cycle are of nonmarine eolian (dune and interdune) and fluvial (channel and floodplain) origin. The mixed nonmarine and shallow marine nature of the inferred paleoenvironments of the lower Cutler beds is significant because the preserved expression of the nonmarine eolian and fluvial units in each cycle demonstrates an external climatic control on system development, whereas the marine units in each cycle (together with associated transgressive and regressive surfaces) demonstrate repeated episodes of relative sea-level change. Key stratigraphic surfaces have been traced from the continentally dominated northeastern portion of the basin into apparently contemporaneous, shallow-marine dominated sections in the southwest of the basin. Interdependent changes in both climate and relative sea-level change are shown to have generated the preserved cycles, which are interpreted to represent high-frequency sequences that are bounded by erosional unconformities (sequence boundaries) and paraconformities (diastems). Relative sea-level lowstand was coincident with climatic aridity at which time exposure of a broad, low-relief coastal plain enabled the generation of a substantial eolian sediment supply that was available for transport by a paleowind aligned parallel to the trend of the paleocoastline, resulting in dune-field construction and accumulation along a coastal belt. Episodes of relative sea-level rise were coincident with a transition to a more humid paleoclimate, during which the eolian sediment supply was greatly reduced, resulting in dune-field deflation and associated expansion of the fluvial system across the deflated former dune-field surface. Marine transgression occurred first in a series of estuarine embayments via the back-flooding of broad but shallow fluvially incised valley systems. Later transgression culminated in marine incursion over the former low-relief floodplain. Episodes of relative sea-level highstand were coincident with maximum climatic humidity. The high-frequency sequences make up a composite sequence set, which itself records a longer-term relative sea-level cycle. At this larger scale, a lowstand systems tract is recorded in the lower part of the studied lower Cutler beds succession, whereas a substantial transgressive systems tract is developed in the upper part. A regionally extensive maximum flooding surface defines the top of the lower Cutler beds. The preserved thickness and number of high-frequency cycles, together with their relation to the long-term subsidence trend of the Paradox Basin, records a periodic switching of both climate and relative sea level, the temporally linked nature of which demonstrates that glacio-eustasy exerted a dominant control on the origin of the preserved succession. The lower Cutler beds preserve evidence for the linkage and feedback between combined climatic and eustatic allogenic controls on sedimentation, and their role in generating high-frequency Milankovitch-style cycles in mixed clastic-carbonate successions